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A new device architecture using giant magnetoresistive sensors demonstrates the capability to detect very low magnetic fields on the pT range. A combination of vertically packed spin-valve sensors with two-dimensional in-plane arrays, connected in series and in parallel, delivers a final detection level of 360 pT/ Hz at 10 Hz at room temperature. The device design is supported by an analytical model developed for a vertically packed spin-valve system, which takes into account all magnetic couplings present. Optimization concerning the spacer thickness and sensor physical dimensions depending on the number of pilled up spin-valves is necessary. To push the limits of detection, arrays of a large number of sensing elements (up to 440,000) are patterned with a geometry that improves sensitivity and in a configuration that reduces the resistance, leading to a lower noise level. The final device performance with pT detectivity is demonstrated in an un-shielded environment suitable for detection of bio-signals. [ABSTRACT FROM AUTHOR]

Magnetoresistive (MR) sensors have been identified as promising candidates for the development of high-performance magnetometers due to their high sensitivity, low cost, low power consumption, and small size. The rapid advance of MR sensor technology has opened up a variety of MR sensor applications. These applications are in different areas that require MR sensors with different properties. Future MR sensor development in each of these areas requires an overview and a strategic guide. An MR sensor roadmap (non-recording applications) was therefore developed and made public by the Technical Committee of the IEEE Magnetics Society with the aim to provide an research and development (R&D) guide for MR sensors intended to be used by industry, government, and academia. The roadmap was developed over a three-year period and coordinated by an international effort of 22 taskforce members from ten countries and 17 organizations, including universities, research institutes, and sensor companies. In this paper, the current status of MR sensors for non-recording applications was identified by analyzing the patent and publication statistics. As a result, timescales for MR sensor development were established and critical milestones for sensor parameters were extracted in order to gain insight into potential MR sensor applications (non-recording). Five application areas were identified, and five MR sensor roadmaps were established. These include biomedical applications, flexible electronics, position sensing and human–computer interactions, non-destructive evaluation and monitoring, and navigation and transportation. Each roadmap was analyzed using a logistic growth model, and new opportunities were predicted based on the extrapolated curve, forecast milestones, and professional judgment of the taskforce members. This paper provides a framework for MR sensor technology (non-recording applications) to be used for public and private R&D planning, in order to provide guidance into likely MR sensor applications, products, and services expected in the next 15 years and beyond. [ABSTRACT FROM AUTHOR]

Eddy current testing (ECT) is a high impact technology to detect surface flaws. However, its reliability is reduced for low conductive titanium alloys, which are broadly integrated into the industry. ECT tools based on magnetoresistive (MR) sensors offer advantages over inductive sensors due to an enhanced spatial resolution, high sensitivity, and bandwidth. In this paper, we demonstrate successful non-destructive testing inspection of titanium surfaces, not achieved by other ECT tools. Here, single surface defect (length = 0.6 mm; width =100~\mu \textm ; depth =50~\mu \textm ) in a non-polished TA6V \alpha titanium mock-up was measured using an MR-ECT probe. The detection was made by an array of magnetic tunnel junctions with 50 \times 50~\mu \text{m}^{\mathrm { {2}}} with optimized field detectivity. A differential measurement employing a heterodyne technique isolated the magnetic field component ( f_{H}- f_{\mathrm { {bias}}} = 1 kHz) from the electric biasing component ( f\mathrm { {bias}} = 4999 kHz) and electromagnetic coupling ( fH = 5 MHz), revealing a bipolar defect signature of 6.01~\mu \textV\text {0-p} amplitude. [ABSTRACT FROM PUBLISHER]

The reduction of the noise level of magnetoresistive (MR) micrometric sensors is often achieved by increasing the sensor area and/or connecting MR elements in large arrays. This paper introduces an innovative strategy to increase the sensing area without increasing the area occupied by the MR sensors on a wafer, therefore minimizing the sensor footprint. Here, the idea is to deposit vertically spin-valve (SV) films, separated by an insulator. This packed structure is then incorporated into arrays, connected in parallel and series, as a general noise reduction strategy. Using calculations, the number of SVs vertically packed ( Z ), combined in series ( X elements) and parallel ( Y elements) was investigated, so that the optimum geometry, which minimizes the field detectivity, can be obtained. The magnetic field detection of SV sensors in the thermal noise limit can be improved from 39 pT/Hz for Z = 1 with a total number of 500 SVs (distributed in plane) to 12 pT/Hz ^1/2 for Z =10 with a total number of 5000 SVs. Besides, the device footprint is also demonstrated to decrease from 0.7 mm2 ( $Z = 1$ ) to 0.07 mm2 ( $Z = 10$ ). These limits of detection are competitive with single SV sensors using magnetic flux concentrators, with the advantage of a high spatial resolution with fewer fabrication steps (at least three). [ABSTRACT FROM PUBLISHER]

The bacterial infection degree of a real biological sample can be determined by using magnetoresistive (MR) cytometry. In this case, it is required to count detection events and to precisely determine the bacterial cells concentration detected by the sensor in each detection event. Here, we are proposing a method to evaluate the bacterial concentration from experimental obtained data. By simulating and analyzing the signals created by clusters with a different number of cells, shapes, and orientations relative to the sensor sensitive direction, clusters that can fit the experimental signals are found. The simulation results are compared with the experimental obtained signals in order to quantify the number of cells that are detected by the MR sensor in different detection events. [ABSTRACT FROM PUBLISHER]

This paper reports on the integration of magnetic tunnel junction (MTJ) sensing devices with magnetoresistance responses above 150% on flexible substrates. The sensors are integrated in a fabrication process based on polyimide materials, and demonstrate sensitivities up to 250~\mu \textV /Oe for bias currents in the 100- \mu \textA range. Assessment of the sensors performance is done under controlled mechanical load conditions, and the magnetoresistance is only slightly affected (1% variation) while sensitivity changes by 7.5% when the bending radii reduce down to 5 mm. The results demonstrate the high potential of the MTJ sensors to be used in applications requiring bending and conforming to non-planar geometries, bringing magnetoelectronic technologies to hard-to-reach regions of space. [ABSTRACT FROM PUBLISHER]

Microvesicles (MVs) are a promising source of diagnostic biomarkers which have gained a wide interest in the biomedical and biosensing field. They can be interpreted as a “fingerprint” of various diseases. Nonetheless, MVs implementation into clinical settings has been hampered by the lack of technologies to accurately characterize, detect and quantify them. Here, we report the specific sensing and quantification of MVs from endothelial cells using a portable magnetoresistive (MR) biochip platform, in less than one hour and within physiologically relevant concentrations (1 × 108 MVs per ml). MVs were isolated from both endothelial and epithelial cells undergoing apoptosis, and characterized by atomic force microscopy (AFM) and nanoparticle tracking analysis (NTA), which revealed similar MV sizes. Importantly, our results showed that the two distinct MV populations could be discriminated with the MR biochip platform, with over a 5-fold capture efficiency of endothelial MVs in comparison to the control (epithelial MVs). Also, unspecific binding of MVs to BSA was less than 1% of the specific signal. The detection strategy was based on a sandwich immunoassay, where MVs were labelled with magnetic nanoparticles (MNPs) functionalized with Annexin V and then captured by anti-CD31 antibodies previously immobilized on the surface of the sensor. Results suggest that this approach allows the detection of specific MVs from complex samples such as serum, and highlight the potential of this technology to become a suitable tool for MVs detection as a complementary method of diagnosis. [ABSTRACT FROM AUTHOR]

As single tunneling magnetoresistive (TMR) sensor performance in modern high-speed applications is limited by breakdown voltage and saturation of the sensitivity, for higher voltage applications (i.e., compatible to 1.8 V, 3.3 V or 5 V standards) practically only a series connection can be applied. Thus, in this study we focused on sensitivity, 3 dB bandwidth and sensitivity-bandwidth product (SBP) dependence on the DC bias voltage in single and series-connected TMR sensors. We show that, below breakdown voltage, the strong bias influence on sensitivity and the 3 dB frequency of a single sensor results in higher SBP than in a series connection. However, the sensitivity saturation limits the single sensor SBP which, under 1 V, reaches the same level of 2000 MHz·V/T as in a series connection. Above the single sensor breakdown voltage, linear sensitivity dependence on the bias and the constant 3 dB bandwidth of the series connection enable increasing its SBP up to nearly 10,000 MHz·V/T under 5 V. Thus, although by tuning bias voltage it is possible to control the sensitivity-bandwidth product, the choice between the single TMR sensor and the series connection is crucial for the optimal performance in the high frequency range. [ABSTRACT FROM AUTHOR]

This paper describes how magnetoresistive materials can be optimized and integrated in magnetic field sensor devices. Giant magnetoresistive (GMR) and tunnel magnetoresistive (TMR) can provide advantageous solutions for a wide range of applications, of which some are described here. Considerations on field sensitivities, intrinsic noise mechanisms, biasing, and integration schemes are presented. [ABSTRACT FROM PUBLISHER]

In this paper, we describe how magnetoresistive sensors can be integrated with microelectromechanical systems (MEMS) devices enabling the mechanical modulation of DC or low frequency external magnetic fields to high frequencies using MEMS structures incorporating magnetic flux guides. In such a hybrid architecture, lower detectivities are expected when compared with those obtained for individual sensors. This particularity results from the change of sensor's operating point to frequencies above the 1/f noise knee. [ABSTRACT FROM AUTHOR]

This paper reports on the development and the fabrication of a novel atomic force microscopy cantilever with integrated magnetoresistive sensors at its tip for scanning magnetoresistance applications. A 2-D scan over an area of 100\times 100~\mu \textm^2 simultaneously generates topographic and magnetic maps with a spatial resolution better than 1 \mu \textm . The magnetic field is detected using two spin valve sensors with a field sensitivity of 61.33 \mu \textV /Oe. The device operation is demonstrated with a ferromagnetic test feature of 1\times 20~\mu \textm^2 . The results demonstrate that this scanning technique is capable of accurately quantifying the absolute magnitude of magnetic stray fields. [ABSTRACT FROM AUTHOR]

Flow cytometers have been optimized for use in portable platforms, where cell separation, identification and counting can be achieved in a compact and modular format. This feature can be combined with magnetic detection, where magnetoresistive sensors can be integrated within microfluidic channels to detect magnetically labelled cells. This work describes a platform for in-flow detection of magnetically labelled cells with a magneto-resistive based cell cytometer. In particular, we present an example for the validation of the platform as a magnetic counter that identifies and quantifies Streptococcus agalactiae in milk. [ABSTRACT FROM AUTHOR]

In this work an alternative neuroscience tool for electromagnetic measurements of neurons at the level of individual cells is developed. To perform such measurements we propose the integration of an array of magnetoresistive sensors on micro-machined Si probes capable of being inserted within the brain without further damage. Si-etch based micromachining process for neural probes is demonstrated in the manufacture of a probe with 15 magnetoresistive sensors in the tip of each shaft. Magnetic tunnel junction sensors with dimensions of 30\ \mum\times 2\ \mum, sensitivities of 3.32 V/T and detectivity of \approx13 nT/Sqrt (Hz) are placed in the end of the sharply defined probe tips. In order to measure the small signals coming from the neurons, a homemade signal amplifying system was used with a noise level of 240 nVRMS for the system bandwidth. The full system noise is 2772 nVRMS. [ABSTRACT FROM AUTHOR]

A spin-valve sensor biochip was used to detect cystic fibrosis related DNA targets for the purpose of developing an affordable diagnostic chip. The strategy used was based on the ac field focusing of magnetically labeled target DNA at sensor sites using U-shaped current lines. U-shaped spin-valve sensors, fabricated within the line structures, detected in real-time the hybridization of DNA targets to complementary DNA probes, previously immobilized onto the chip surface. Hybridization occurred in relatively short times (15-30 min) in comparison with conventional hybridization approaches (3 to 12 h). Statistical data on detection signals for single probe and multiprobe experiments was obtained, showing a significant difference between complementary binding signals and noncomplementary and background ones. [ABSTRACT FROM AUTHOR]

On-chip spin valve sensors were used to measure the flow velocity within fabricated poly(dimethylsiloxane) (PDMS) microchannels. Superparamagnetic particles with dimensions of 250 nm in diameter were introduced into a 100 μm wide microchannel, bonded to the chip surface. The flow velocity was determined by measuring the changes in resistance of a pair of spin-valve sensors, 1.65 mm apart from each other, caused by the passage of ensembles of superparamagnetic particles, flow velocities ranging from 50 to 300 μm/s were estimated, depending on the pressure used to drive the flow. The application of this system provides a means to measure and control flow velocities on microfluidic devices. [ABSTRACT FROM AUTHOR]

Highlights • Thermal stability of H ex for MnPt and MnIr was studied in AF/FM bilayers and TMR stacks. • A model was developed to describe the H ex and H c behaviors at high temperatures. • A better thermal stability was observed in TMR stacks than bilayers. Abstract Ensuring a reliable reference in magnetoresistive devices requires robust exchange biased antiferromagnetic/ferromagnetic (AF/FM) bilayers with strong resilience to high temperatures. In this work, we studied the thermal stability of MnPt and MnIr by investigating the exchange bias effect in AF/CoFe bilayers and tunneling magnetoresistance (TMR) full stacks. In bilayers, the obtained blocking temperature ( T b ) of 380 °C for MnPt is higher than T b of 340 °C for MnIr, indicating a better thermal stability. Based on Malozemoff's model, we developed a comprehensive model to describe the exchange bias field ( H ex ) behavior at high temperature by considering the T b distribution and temperature dependence of FM magnetization, giving a good agreement with the experimental data. With increasing temperature, a coercivity ( H c ) enhancement peak was observed in MnIr bilayer, which shows a good agreement with T b distribution. This evidences the H c peak is dominated by the thermal activation of AF grains, indicating that the H c peak only appears at T b center , which is lower than overall T b . The structural properties and grain size of AF layers were studied by X-ray diffraction (XRD), where a mean grain size of 10.6 and 12.6 nm were obtained for MnPt and MnIr, respectively. Compared to bilayers, a more stable thermal behavior was observed in full stacks. In conclusions, our study compared the thermal stability of exchange bias coupling using MnPt and MnIr and developed a comprehensive model to study their temperature dependence. This proposed approach can be used for theoretical studies and experimental designs of AF-based materials for spintronic applications, such as magnetoresistive devices and high density memories. [ABSTRACT FROM AUTHOR]

Highlights: [•] A new eddy current probe was designed, simulated and experimentally validated. [•] Two techniques were applied to improve measurements at high frequencies. [•] A comparison between the measurement techniques was performed. [•] Testing on a welding joint demonstrated the ability to detect 400μm crack defects. [Copyright &y& Elsevier]